I wouldn&#39;t really want to use too small a sail on that Isonic. 6-6.5m is pretty much optimum in decent powered conditions. You could use a 5m but this would feel a little small for an 87 board. I&#39;ve used the 87 with 6m and this combination felt ideal. With my other slalom board (Falcon 90). The X15 5.2 was a little small and did not feel great, although useable.
6m should be great with plenty of downhaul and a small fin in winds upto 30 knots, provided it&#39;s not too rough, after that I would certainly be looking for a smaller board if not before.